The present invention relates to a manufacturing method of elbows made of cast stainless steel, such an elbow provided at its ends with straight tube portions, for use in piping applications involving high temperatures and pressures and severe corrosion, and it provides a method for refining the structure of the straight tube portions at the ends of the elbow.
For piping elbows for use at high temperatures and pressures and under highly corrosive conditions, as in atomic power plants or in petrochemical plants, etc., for example, austenitic stainless steels, such as 18-8 series stainless steels, are now frequently in use. Such elbows may be manufactured either by casting or by forging.
Of these two methods, the use of casting is advantageous in that the adjustments of alloy components to meet the applications requirements are simplified, and that arbitrary shapes required for pipings may be produced. That's why this method is very popular, but such cast stainless steels are disadvantageous in that their crystal grains become coarse, giving rise to a low response to the ultrasonic-flaw test (hereinafter referred to as UT). This leads to a deplorable situation today when the growing importance of the in-service inspection at real plants (hereinafter referred to as ISI), for example, is recognized. Thus low response to the UT hinders adequate inspections for flaws such as cracks, etc., causing uncertainty in ensuring safety. As a countermeasure, crystal grain refinement by way of die casting, etc., are being tried. However, their inadequacy, as compared with the use of forging, can hardly be denied.
On the other hand, in the case of forging, due to the refinement of crystal grain, the response to UT in ISI is improved, but its use involves a variety of problems. Generally, austenitic stainless steels (e.g., AISI 304 or 316), having single phase structures of austenite, are highly susceptible to inter-granular corrosion (hereinafter referred to as IGC) or stress corrosion cracking (hereinafter referred to as SCC) where they are exposed to the welding heat. Actually, their use is not prohibited, but is greatly restricted. Where forging is applied, the methods commonly employed in manufacturing elbows are by the mandrel method or by assembling by welding after bending a plate. By these methods, the elbow 1 is bent from end to end; such an elbow 1 as shown in FIG. 2 which has straight tube portions 2 at both ends thereof can not be obtained. This straight tube portion 2 at the elbow end, as it is welded at 4 to the straight pipe 3, is the indispensable part in ensuring the accuracy in ISI by way of UT. Accordingly, a forging showing high response to UT will fail to meet the applications requirements, if it has no straight tube portions 2 formed thereon.
As described in the foregoing, in actual situations with piping elbows for uses involving high temperatures and pressures and corrosion conventional castings and forgings, having both merits and demerits, have given no assurances for proper safety (potential for accident prevention and high response to UT in ISI) under rigorous service conditions.